Photocatalytic reactions are reactions in which the energy input is provided, at least in part, by light and which are accelerated by a catalyst. Photocatalytic reactions are generally understood to comprise a sequence of processes initiated by absorption of a photon by the photocatalyst. This causes promotion of electrons from the catalyst's valence band to the conduction band, creating electron-hole pairs. These cause the catalysis through redox reactions.
The archetypal photocatalytic reaction is the catalysed photolysis of water, which was first reported in 1972 using a titanium dioxide photoelectrode (A Fujishima and K Honda, Nature, 1972, 238, 37-38).
Since at least the report of the photocatalytic hydrolysis of water by Fujishima and Honda, it has been understood that photocatalytic reaction is require the use of semiconductors, such as titanium dioxide, having proper band gaps and band edges, that is to say non-overlapping valence and conduction bands. Accordingly, photon energies must be greater than the band gap in order to generate the desired electron-hole pairs. Innumerable semiconductors have been evaluated for utility as photocatalysts.
Light absorption across the bandgap in semiconductors is exploited in many important applications such as photovoltaics, light-emitting diodes and photocatalytic conversion. Metals differ from semiconductors in that there is no bandgap separating occupied and unoccupied levels with a continuum of energy levels across the conduction band. For this reason, whilst the creation of electron-hole pairs will occur upon absorption of photons by metals, the overlapping nature of the valence and conductance bands lead to recombination. This prevents the harnessing of the electron-hole pairs in photocatalytic reactions.
It is nevertheless possible to excite electrons from states in a fully occupied valence band into unoccupied levels higher than the conduction band edge, a phenomenon evidenced by strongly coloured metallic materials. However, the utility of such electron-hole pairs in light-harvesting or similar applications has been assumed to require separation of the created carriers in the conductor using an electric field. Such a field is typically found at an interface in a semiconductor, such as a p-n junction in photovoltaics or at its surface in photocatalysis. However, the high conductivity of a metal would preclude sufficient electric field being available to separate photocarriers, meaning catalytic photocatalytic activity is not a property metals are anticipated to possess.
For the foregoing reasons, semiconductors are universally utilised in photochemical/photovoltaic applications. However, the use of materials with defined band gaps generally leads to poor light absorption and thus low efficiencies in photocatalytic reactions and other photovoltaic applications. Moreover, conductive glasses such as tin oxide are generally used in photoelectrochemistry as a substrate material for semiconductors. However, this approach is disadvantageous, inevitably increasing expense and/or complexity.